Thin film transistors are fundamental components in modern-age electronics, including, for example, sensors, and imaging and display devices. Thin film transistor circuits using current mainstream silicon technology are too costly, particularly for large-area device applications such as backplane switching circuits for displays like active matrix liquid crystal monitors or televisions, where high switching speeds are not essential. The high costs of silicon-based thin film transistor circuits are due to the capital-intensive silicon fabrications as well as the complex high-temperature, high-vacuum photolithographic fabrication processes under strictly controlled environments.
Because of the cost and complexity of fabricating silicon-based thin film transistor circuits using conventional photolithography processes, there has been an increased interest in plastic thin film transistors which can potentially be fabricated using solution-based patterning and deposition techniques, such as spin coating, solution casting, dip coating, stencil/screen printing, flexography, gravure, offset printing, ink jet-printing, micro-contact printing, and the like, or a combination of these processes. Such processes are generally simpler and more cost effective compared to the complex photolithographic processes used in fabricating silicon-based thin film transistor circuits for electronic devices. To enable the use of these solution-based processes in fabricating thin film transistor circuits, solution processable materials are therefore required.
Most of the current materials research and development activities for plastic thin film transistors has been devoted to semiconductor materials, particularly solution-processable organic and polymer semiconductors. Other material components, such as solution processable dielectric materials, have not been receiving much attention.
For plastic thin film transistor applications, it is desirable for all of the materials be solution processable. It is also highly advantageous to fabricate the materials on plastic substrates at a temperature of less than about 200° C., and for example less than about 150° C. The use of plastic substrates, together with flexible organic or polymer transistor components can transform the traditional thin film transistor circuits on rigid substrates into mechanically more durable and structurally flexible plastic thin film transistor circuit designs. Flexible thin film transistor circuits may be useful in fabricating mechanically robust and flexible electronic devices.
Generally, the dielectric layer which serves as the gate dielectric in a thin-film transistor should have at least one of the following properties: i) a smooth uniform layer without pinholes, ii) a high dielectric constant to enable the thin film transistor to operate at lower voltages, and iii) no adverse effects on the transistor's performance. Additionally, for flexible integrated circuits on plastic substrates, the dielectric layer should be prepared at temperatures that would not adversely affect the dimensional stability of the plastic substrates, i.e., generally less than about 200° C., for example less than about 150° C.
A wide variety of organic and polymer materials, including polyimides (F. Garnier, et. al. J. Am. Chem. Soc. 1993, Vol. 115, pp. 8617), poly(vinylphenol) (M. Halik, et al. J. Appl. Phys. 2003, Vol. 93, pp. 2977), poly(methyl methacrylate) (J. Ficker, et. al. J. Appl. Phys. 2003, Vol. 94, p. 2638), polyvinylalcohol (R. Schroeder, et. al. Appl. Phys. Lett. 2003, Vol. 83, pp. 3201), poly(perfuoroethylene-co-butenyl vinyl ether) (J. Veres, et al. Adv. Funct. Mater. 2003, Vol. 13, pp. 199), and benzocyclobutene (L.-L. Chua, et. al. Appl. Phys. Lett. 2004, Vol. 84, p. 3400), have been studied as dielectric layers. These materials, however, do not generally meet all the economic and/or functional requirements of low-cost thin film transistors. In particular, most of the organic or polymer dielectric materials generally have low dielectric constants, and thus cannot enable low-voltage electronic devices.